Numerous fields of modern technology require that particles be removed from fluid. Such separation processes permit either the liquid phase or particulate matter to be recycled. In biotechnology, biological particles have to be removed from medium containing the product. Continuous stirred suspension bioreactors are operated with continuous addition of fresh medium and removal of spent medium, often containing the desired product. A number of systems have been developed to increase the cell density and productivity in these bioreactors by recycling cells from the spent medium stream. Of these systems, spin filters are most widely commercially available. Spinning (rotation) of these cylindrical filters inside bioreactors helps reduce the fouling of the filter surface. Nonetheless, the usefulness of these existing systems is limited by progressive protein and cellular fouling of the filters. Filter fouling and the build-up of nonviable cells in the suspended phase of spin-filter bioreactors generally limits the time of productive operation. The scale-up of spin-filters has been limited because, as the volume of bioreactors increases, it is not practical to provide enough filter area inside the reactor to maintain operation. Cross-flow, sedimentation and centrifugation systems are limited by fouling and scalability. They have not been widely used in industry. Centrifugation systems also have not been widely used, possibly because of the high cost.
Recently, great effort has been directed at the development of acoustic separation methods to replace or enhance conventional technologies. The establishment of a standing wave in the fluid results in the formation of velocity nodes or antinodes to which the particles are forced to migrate by the radiation force, depending on their compressibility and density. (Most solid and liquid particles move toward the velocity antinodes.) Nodes and antinodes are at right angles to the direction of propagation of the sound waves, and the nodes are spaced from adjacent nodes by a distance equal to one half of the wavelength of the acoustic wave. The aggregating effect of ultrasonic sound within these antinodes has already been described in the literature. From E. Skudrzyk, "Die Grundlagen der Akustic," Springer Verlag, Wien, 1954, S. 202-205, S. 807-825; L. Bergmann, "Der Ultraschall und seine Anwendungen in Wissenschaft und Technik," Verlag hirzel, Zuerich, 1954: as well as K. Asai and N. Sasaki, "Treatment of slick by means of ultrasonics," Proceedings of the 3rd International Congress on Coal Preparation, Institut National de l'Industrie Charbonniere, Brussels-Liege, 1958, it follows that the frequency to be used in the applied sound is best chosen within the magnitude of the so-called characteristic frequency f.sub.0, which can be calculated from ##EQU1## whereby .eta. constitutes the kinematic viscosity and r the radius of the particle. Using this frequency range, the effect of radiation force and cumulative acoustically induced Bernoulli forces within the antinode planes can be maximized.
According to U.S. Pat. No. 4,055,491, ultrasonic standing waves are used to flocculate small particles, such as blood or algae cells, within the velocity antinodes of the acoustic field so that they settle out of the carrying liquid by gravity. But the undefined placement of the ultrasonic source and therefore low efficiency of the standing wave field due to undefined resonance boundary conditions result in high energy losses due to a considerable fraction of traveling waves. The described process is limited to discontinuous operations. The apparatus presented in U.S. Pat. No. 5,164,094 mainly modifies the geometry compared to the embodiments described in U.S. Pat. No. 4,055,491. However, a considerable portion of energy is still lost since frequencies of the sound field applied to the vessels carrying the dispersion is not controlled by well-defined resonance boundary conditions.
An embodiment to separate particles with various acoustic qualities is described in U.S. Pat. No. 4,523,682. A low resonance mode of a vessel containing a dispersion is excited by a relatively small transducer mounted at one end of the vessel, resulting in node and antinode planes perpendicular to the transducer/vessel interface. Perpendicular modes created by the acoustic point source mean that the system cannot be described as a one-dimensional resonator. The fraction of attenuated traveling waves in the longitudinal direction is high compared to the accumulated acoustic energy in the transversal standing wave field. Acoustic attenuation results in a temperature increase within the dispersion along the flow direction. Temperature changes affect sound velocity and resonance frequency, and cause a non-homogeneous temperature distribution along the flow direction which decreases the resonance quality of the field. As a result, the treatment period necessary to achieve the desired separation is prolonged.
Because of the long acoustic treatment periods necessary to achieve aggregation and sedimentation of the particles captured in the antinode planes, efforts were undertaken to move the antinodes of a standing wave relative to the dispersion, in order to obtain the desired separation effect directly by utilizing acoustic forces alone. U.S. Pat. No. 4,673,512 introduces an interference standing wave field generated by opposing transducers which are excited with the same frequency. By controlling the phase shift between the electric excitation signals of the two acoustic sources, it is possible to move particles trapped within the antinodes or nodes of the traveling interference pattern in the dispersion. Using this method, a relatively short treatment period can be achieved. The disadvantage of this method is its non-resonant nature. Much more energy is used to maintain an interference standing wave field compared to a resonant standing wave field of the same amplitude. The result is higher electrical power consumption and thermal dissipation for producing a given acoustic particle velocity amplitude. The same problem has to be considered in U.S. Pat. No. 4,759,775, in which only the method of creating the traveling interference pattern is different.
U.S. Pat. No. 4,877,516 introduces the idea of the controlled movement of local gradients of the acoustic amplitude of the standing field perpendicular to the direction of sound propagation. Thus, particles are moved within the antinodes or nodes of the field by the Bernoulli-force which is directly related to described gradients and is acting parallel to the antinode planes. The disadvantage of the embodiment is the requirement of mechanically moving array to produce acoustic shadows in order to achieve the desired movement of local gradients of the standing wave.
Stepwise movement of the antinodes of a resonant standing wave by exciting succeeding resonance modes of the resonator system is described in the PCT Appl. No. PCT/AT89/00098. Although resonance boundary conditions are fulfilled in some of the described embodiments, there is still considerably acoustically induced dissipation due to the resonator frequencies used, which have always been chosen very close to an Eigen-frequency of the transducer in the past.